Input parasitic metal detection

ABSTRACT

A system and method of controlling inductive power transfer in an inductive power transfer system and a method for designing an inductive power transfer system with power accounting. The method of controlling inductive power transfer including measuring a characteristic of input power, a characteristic of power in the tank circuit, and receiving information from a secondary device. Estimating power consumption based on the measured characteristic of tank circuit power and received information and comparing the measured characteristic of input power, the information from the secondary device, and the estimated power consumption to determine there is an unacceptable power loss. The method for designing an inductive power transfer system with power accounting including changing the distance between a primary side and a secondary side and changing a load of the secondary side. For each distance between the primary side and the secondary side and for each load, measuring a circuit parameter on the primary side in the tank circuit and a circuit parameter on the secondary side during the transfer of contactless energy. The method further including selecting a formula to describe power consumption in the system during the transfer of contactless energy based on coefficients and the circuit parameters, and determining the coefficients using the measured circuit parameters.

FIELD OF THE INVENTION

The present invention relates to power loss accounting for a contactlesspower supply system.

BACKGROUND OF THE INVENTION

It is becoming more common for contactless power to be transmitted toportable electronic devices, for example by utilizing inductivecoupling. Many inductive power supply systems suitable for poweringportable devices include two main components: (1) an inductive powersupply or primary unit having at least one primary coil, through whichit drives an alternating current, creating a time-varyingelectromagnetic field, and (2) a portable electronic device or secondarydevice, separable from the primary unit, including a secondary coil,which when placed in proximity to the time-varying field, the fieldinduces an alternating current in the secondary coil, therebytransferring power from the primary unit to the secondary unit.

Contactless power supply systems are not 100% efficient. That is, someenergy is lost in order to transfer power from the primary unit to thesecondary unit. For example, some losses may be caused by the switchingcircuit components and other losses may be caused by the primary coil,sometimes referred to as ohmic losses, which are proportional to theohmic resistance in the components and to the square of the currentrunning through them. Foreign objects, and especially metal foreignobjects, can also affect efficiency and in some cases cause a safetyconcern. Metal placed in the field is sometimes referred to as parasiticmetal. Some parasitic metal in the field may be acceptable, for example,many portable devices, even ones powered by contactless power supplysystems, sometimes include metal. The acceptable metal is sometimesreferred to as known or friendly parasitic metal.

Some systems and techniques have been developed to attempt to detectwhether there is an unacceptable amount of parasitic metal in the field.One basic system includes a power consumption detector in the electriccircuit of a power sending terminal. When a piece of metal is placed onthe power sending terminal instead of a portable device, the amount ofconsumed power at the power sending terminal increases abnormally. Inorder to prevent this abnormality, the power consumption detectormeasures the amount of power consumed by the power sending terminal.When the measured amount of the consumed power reaches a predeterminedupper threshold, it is determined that there is an unusual situation andtransmission of power is suppressed. Although a system such as thisprovides basic parasitic metal detection, it has flaws. For example, thesystem cannot account for (1) friendly parasitic metal, (2) portabledevices that consume different amounts of power, or (3) power losses dueto the misalignment of the power sending terminal and the portabledevice.

Other parasitic metal detection techniques have also been developed. Forexample, some systems can account for (1) the power being supplied tothe actual load of the secondary device, (2) the friendly parasitics ofthe secondary device, (3) situations where there is not a simple 1:1relationship between the primary unit and the secondary device, or (4)situations where presence of the secondary device does not necessarilyphysically exclude all foreign objects. Some of these techniques involvedisconnecting the secondary load or communicating information from thesecondary device to the primary unit. A number of these techniques aredescribed in U.S. Patent Publication 2007/0228833 to Stevens, filed onMay 11, 2005 entitled “Controlling Inductive Power Transfer Systems”which is herein incorporated by reference in its entirety.

Although some previous systems can provide parasitic metal detection, insome situations these systems can be inadequate. For example, knownsystems do not account for the known losses accurately enough andtherefore incur too many false positives that result in a systemrestriction or shut down. To put it another way, one issue with someknown parasitic metal detection systems is that their resolution isloose enough that a piece of metal could heat up to an undesired level.Utilizing a method that has an improved resolution or accuracy to detectlosses can address this and other issues.

SUMMARY OF THE INVENTION

The present invention provides a contactless power supply systemincluding a primary unit and a secondary device in which parasitic metalin proximity to the primary unit can be more accurately detected byaccounting for changes in known power losses during operation. Theamount of power loss during inductive power supply transfer in aninductive power supply system can vary depending on the alignment of theprimary unit and the secondary device. Further, the amount of power lossduring inductive power supply transfer can also vary as a function ofchanges in the operating frequency of the switching circuit in theprimary unit or as a function of changes in the secondary device load.By accounting for changes in known power losses during operating, a moreaccurate determination of the amount of unknown power loss can be made.Further, the secondary measurements and primary measurements can besynchronized to increase accuracy. The more accurate the determinationof unknown power losses, the more false positive parasitic metaldetections that can be avoided. Further, the sooner (both in time andpower threshold) a true positive can be detected.

In one embodiment, the present invention provides a contactless powersupply system in which parasitic metal can be detected by comparing anexpected input to a measured input. In the current embodiment, theexpected input is determined as a function of various known losses inthe system, including losses due to the misalignment of the primary unitand the secondary unit. The expected input does not account for anyparasitic metal in the field, so if there is parasitic metal in thefield, the expected input will be different from the measured input.

In one embodiment, a system and method for controlling a contactlesspower transfer system is provided. The contactless power supply systemincludes a primary unit with switching circuitry and a tank circuitoperable to generate an electromagnetic field and at least one secondarydevice, separable from the primary unit, and adapted to couple with thefield when the secondary device is in proximity to the primary unit sothat power can be received inductively by the secondary device from theprimary unit without direct electrical contacts. The primary unitincludes among other circuitry, a controller, an input measurement unitlocated before the switching circuitry and a tank measurement unitlocated after the switching circuitry. The portable device includesamong other circuitry, a secondary measurement unit and a controller.From time to time, measurements are transmitted from the secondarydevice to the contactless power supply where they are used by thecontroller together with measurements from the coil measurement unit todetermine an expected input. The primary and secondary measurements canbe synchronized, for example by accounting for the time it takes to takeand send the measurement, time stamping the measurements, or utilizing aweighted average or other synchronization technique. The expected inputis compared to the actual input to determine the amount of parasiticmetal present in the field. The contactless power supply system can takea variety of actions in response to the detection of parasitic metal,for example restricting or stopping the supply of contactless power.

It can be difficult to distinguish between losses resulting in increasedohmic losses due to misalignment and losses due to parasitic metal inthe field. This is generally because the input current is typicallyaffected by both. However, losses resulting from a reduction in couplingand losses due to parasitic metal do not affect the primary coil currentin the same manner. Leveraging this difference, a prediction functionincluding both a characteristic of input power and a characteristic ofprimary unit coil power can determine whether or not there is a foreignobject present in proximity to the primary unit. One advantage of thepresent invention is that it can distinguish between losses due tocoupling and losses due to parasitic metal, making it possible to avoidfalse positives of parasitic metal detection in some situations.

These and other features of the invention will be more fully understoodand appreciated by reference to the description of the embodiments andthe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a block diagram of a contactlesspower supply system capable of input parasitic metal detection.

FIG. 2 illustrates one embodiment of a method of input parasitic metaldetection.

FIG. 3 is a diagram showing a representative graphs of input power andpower consumption in a variety of different scenarios.

FIG. 4 is a representative diagram of a geometric positioning system foruse in calibrating the input parasitic metal detection system.

Before the embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Also, it is to be understood that thephraseology and terminology used herein are for the purpose ofdescription and should not be regarded as limiting. The use of“including” and “comprising” and variations thereof is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items and equivalents thereof.

DESCRIPTION OF CURRENT EMBODIMENTS

The present invention is directed to systems and methods for accountingfor power loss in the system and understanding whether unaccounted forlosses are detrimental to operation. For example, their may be parasiticmetal, a damaged component, or something else in the electromagneticfield causing power loss. In one embodiment, primary coil current,secondary current, and secondary voltage are utilized to determine anexpected primary input current. When the expected primary input currentis properly determined, it can be compared to the measured primary inputcurrent in order to detect whether and in some embodiments, how much,unaccounted power loss is present.

Input current varies with the power lost or consumed in the contactlesspower supply system. For example, the input current is affected byparasitic metal losses, the amount of power delivered to the load,primary and secondary rectification losses, primary switching losses,losses in the tank circuit, losses due to equivalent series resistanceof any resonating capacitor, losses due to poor coupling caused bymisalignment of the portable device and the contactless power supply, aswell as other losses in the system. Measurements of the primary coilcurrent, secondary current from the rectifier, and secondary voltagefrom the rectifier to ground may be utilized along with otherinformation to estimate the various losses in the system that are notdue to parasitic metal. Then, if the expected primary input current doesnot match the measured input current, the system knows there is anunexpected power loss and can conclude that there is a damagedcomponent, parasitic metal or something else in the field causing thepower loss. A parasitic metal or a damaged capacitor, coil, or fieldeffect transistor can be damaged in such a way that it heats up. In someembodiments, the expected primary input current may account for friendlyparasitics, and in other embodiments it may not. In alternativeembodiments, other characteristics of power may be measured in order toaccurately estimate the expected input current or to estimate adifferent expected characteristic of input power that will be useful inparasitic metal detection.

It can be difficult to determine whether certain power losses are due tomisalignment or parasitic metal because it may be difficult todistinguish whether changes in input current are due to parasitic metalbeing placed in the field or increased losses as a result of a change inalignment between the primary unit and secondary device. The inputcurrent may stay relatively the same because the coupling changedsimultaneously as parasitic metal was added to the field. For example,the secondary device could be nudged out of alignment by a user throwingtheir keys next to the secondary device. The parasitic metal in the keysmay counteract some or all of the input current change that would havebeen caused by the misalignment. The contactless power supply system isnot typically aware of the particular alignment between the primary unit10 and the secondary device 30. Instead, the system calculates powerloss by comparing the amount of power transmitted to the amount of powerreceived, and subtracting the known acceptable losses. By utilizing coilcurrent, additional known acceptable losses due to misalignment can beaccounted for.

When there are losses due to parasitic metal in the field, therelationship between input power and expected power is different fromthe relationship between input power and expected power when the primaryunit and secondary unit are misaligned. That is, the relationshipbetween input power and expected power due to misalignment can becaptured in a formula, so that if that when measured, if the measureddata does not fit that formula a determination can be made that there isadditional unknown power loss, for example due to parasitic metal. Thiscan be true for characteristics of input power and expected power aswell. For example, when there are losses due to parasitic metal in thefield, the relationship between input current and coil current isdifferent from the relationship between input current and coil currentwhen there are losses due to poor coupling. Accordingly, by determiningthe expected input current as a function of the coil current, theaccuracy of the parasitic metal detection can be increased. In thecurrent embodiment, losses due to poor coupling were estimated byperforming a best-fit analysis of data that was captured by takingmeasurements as the coupling between the primary unit and secondarydevice was changed. In alternative embodiments, the losses due to poorcoupling may be calculated instead of estimated. For example, where therelative position of the primary unit and the secondary device is known,it may be possible to calculate the power loss due to misalignment. Itis worth noting that in some systems, in response to misalignment, theprimary unit may increase its power, such as by increasing the primarycurrent, with respect to a given load current, this can cause lossesdependent on primary coil current to increase. This can include primaryelectrical losses, coil losses, primary and secondary magnetic losses,and friendly parasitic losses. These changes in losses due to changes inpower level can be accounted for in a formula that maps therelationship. In addition, when the alignment changes, the position ofnot only the secondary coil with respect to the primary coil changes,but also the position of any friendly parasitic metal on the secondarydevice changes. As the amount of electromagnetic field that intersectssecondary shielding or other friendly parasitics changes, the amount offriendly parasitic power loss also changes. All of these changes due toincreased power, changes in alignment, changes in frequency can beaccounted for during the calibration method.

FIG. 1 illustrates parts of an inductive power transfer system embodyingone embodiment of the present invention. The system 100 includes aprimary unit 10 and at least one secondary device 30. The inductivepower transfer system may have a number of suitable configurations. Onesuitable configuration is a power transfer surface where one or moresecondary devices 30 can be placed.

Still referring to FIG. 1, the primary unit 10 is capable of generatingwireless power for transmission to one or more secondary devices. Theprimary unit 10 generally can include an AC/DC rectifier 11, acontroller 16, a switching circuit 14, a tank circuit 23, an inputdetector circuit 24, and a tank detector circuit 26. In this embodiment,the tank circuit 23 includes a primary coil 12 and a capacitor 15;however, the configuration of the tank circuit 23 may vary fromapplication to application. The primary coil 12 may be a coil of wire oressentially any other inductor capable of generating an electromagneticfield that can be received by another inductor. In embodiments that arepowered by AC Mains, power from the AC Mains is rectified by the AC/DCrectifier 11 and used to provide power to various circuitry in theprimary unit and, in conjunction with the controller 16 and theswitching circuitry 14, to generate alternating current in the tankcircuit 23. Although not shown, the primary unit 10 may also include aDC-DC converter in those embodiments where conversion is desired.Alternatively, the system may not be connected to AC Mains. For example,in one embodiment, the system can accept straight DC input with noconverter. The controller 16 is configured to control the timing of theswitching circuitry 14 to create the alternating current in the tankcircuit 23. In some embodiments, the timing of the switching circuitry14 may be controlled to vary the operating frequency of the switchingcircuit at least in part on the basis of feedback from the secondarydevice 30. The controller 16 may include communication circuitry toenable communication with the secondary device 30. The controller 16 maycommunicate by utilizing the inductive coupling, for example by using abackscatter modulation scheme, or by an external communication path suchas an RF transceiver.

The control unit 16 includes a microprocessor in the current embodiment.The microprocessor has an inbuilt digital-to-analogue converter (notshown) to drive the output to the switching circuit 14. Alternatively,an ASIC could be used to implement the control unit 16, as well as someor all of the other circuit elements of the primary unit. Although forsimplicity the communication circuitry is shown in conjunction with thecontroller block, it should be understood that the communicationcircuitry could be separate from the controller circuitry. Further, thecommunication circuitry can utilize the primary coil for communication,or a separate communication path, such as an RF transceiver.

In one embodiment, the system can include a calibration unit. Forexample, the primary unit 10 may include a calibration unit in thecontroller 16, or located elsewhere in the system. Calibration numbersor coefficients can be stored both on the primary side in the controller16 and on the secondary side when power transfer between a particularprimary unit and secondary unit, the calibration data can be combinedtogether in a formula in order to predict whether there are anyunaccounted for losses during operation, such as parasitic metal in thefield.

The calibration unit can store information about the losses in thesystem, for example the losses in the primary unit, secondary unit, orthe coupling between the two. By design, at manufacture, and/orperiodically thereafter, the losses in the primary unit may becalibrated and stored within the calibration unit. The calibration unitsupplies the stored information to the control unit 16 to enable thecontrol unit 16 to use the information in determining whether there isparasitic metal in the field. This calibration unit may vary thecompensation information to cope with variable losses in the primaryunit. The calibration unit may contain data related to the electricaland magnetic losses between the primary and secondary. For example, thecalibration unit may contain data derived from a best-fit analysis ofsweeping the secondary device through a range of different positions onthe primary unit. This best-fit can be distilled into coefficients for aformula to determine an expected input current. For example, in oneembodiment, the formula for expected input current is:

Expected Primary Input Current=0.5*i _(sec)+(0.052*i _(sec) *V_(sec))+(0.018*i _(coil))−0.0009

The Expected Primary Input Current is determined by utilizing a numberof different terms that represent various losses in the system. Forexample, in this embodiment, the 0.5*i_(sec) term accounts for thesecondary rectification losses, the (0.052*i_(sec)*V_(sec)) termaccounts for the power delivered to the load, and the (0.018*i_(coil))term accounts for the power lost in the tank circuit, and the 0.009 termis an offset value.

The i_(sec) and V_(sec) values in the current embodiment are respectivemeasurements of the instantaneous current and voltage after thesecondary AC/DC rectification or the average current and voltage over asimilar predetermined length of time. The i_(coil) value in thisembodiment is a measurement of the peak current in the primary coil.Alternatively, the i_(coil) value could be RMS, peak to peak, or acombination (i.e. crest factor, or a voltage measurement could be usedinstead. All three of these measurements are synchronized in the currentembodiment in order to determine an expected input current at aparticular time. The various coefficients were derived based on datacollected utilizing external current and voltage readings. Inparticular, the coefficients were selected based on a best-fit analysisbased on data of voltage and current readings as the coupling waschanged between the primary and secondary by changing the alignment. Inthe current embodiment, the expected input current was valid forsecondary bridge voltages of 17V-24V and power ranging from 0-60 watts.The coil drive voltage was assumed to be 19V.

In general, the expected primary input current formula is able toaccount for coupling losses because the primary coil current varies withcoil to coil spacing. The coefficients help to track the coil current asthe x and z spacing changes. That is, the coefficients help to track thecoil current as the coils become more horizontally offset, verticallyoffset, and more or less parallel. Essentially, when coupling getsworse, the input current goes up and coil current goes up, but whenparasitic metal is placed in the field input current goes up, but coilcurrent may not go up as much. Put another way, when coupling getsworse, the relationship between input current and coil current orbetween input power and expected power or between a characteristic ofinput power and a characteristic of expected power follows the expectedrelationship. But when parasitic metal is added to the field, therelationship between input current and coil current deviates from thatexpected relationship. In some embodiments, some or all of thecoefficients or information that may be used to derive the coefficientsmay be transmitted from the secondary device to the primary unit.Transmission of this data allows the primary unit to be forwardcompatible for devices that may come out in the future, where differentcoefficients are appropriate.

The primary unit 10 in the FIG. 1 system includes an input detectorcircuit 24 connected to the control unit 16. The input detector circuit24 performs a measurement of a characteristic of the electrical powerdrawn by the switching circuit 14, in response to a signal provided bythe control unit 16 or an internal clock signal. The input detectorcircuit 24 provides an output representative of a characteristic of theelectrical power drawn by the switching circuitry 14 to the control unit16. The input detector circuit 24 in the current embodiment is aninstantaneous current sensor that is capable of sensing the inputcurrent. In general, the input detector circuit 24 may be locatedanywhere in the primary unit before the switching circuitry 14. Inalternative embodiments, the input detector circuit may includeessentially any sensor or sensors capable of measuring one or morecharacteristics of the input power, which can be utilized to determinethe input power or to determine whether parasitic metal is present inthe field or another unexpected loss has occurred. The input detectorcircuit communicates its output with the primary controller 16. Theinput detector circuit output may be time stamped and buffered in theinput detector circuit 24, the controller 16, or elsewhere in theprimary unit 10, to assist with synchronization of other measurements,where appropriate. Additionally, the output can be smoothed or have aconfigurable weighted average applied. In one embodiment there may begaps in the data, because the processor is unable to take a sample, sothose terms can be weighted with zero or a negligible weight. In someembodiments, the secondary device may provide synchronizationinformation or a synchronization standard may be preprogrammed into theprimary unit that describes how or when the secondary device is samplingdata, so that the primary unit can synchronize its measurements. Forexample, the secondary device may not time stamp its data, but ratherprovide a measurement to the primary unit with an expectation that thedata was sampled at a particular time with respect to when it wasreceived.

The primary unit 10 in the FIG. 1 system includes a tank detectorcircuit 26 connected to the control unit 16. The tank detector circuit26 performs a measurement of a characteristic of the electrical powerdrawn by the tank circuit 23, in response to a signal provided by thecontrol unit 16 or an internal clock signal. The tank detector circuit26 provides an output representative of a characteristic of theelectrical power drawn by the tank circuit 23 to the control unit 16.The tank detector circuit 26 in the current embodiment is a peak currentdetector. In general, the tank detector circuit 26 may be locatedanywhere after the switching circuit 14, including the input to the tankcircuit 23, between the primary coil 12 and capacitor 15 of the tankcircuit 23, or after the tank circuit 23. In other embodiments, the tankdetector circuit 26 may include essentially any sensor or sensorscapable of measuring one or more characteristics of the tank circuitpower, which can be utilized to determine the tank circuit power or todetermine whether parasitic metal is present in the field. The tankcircuit detector 26 communicates its output with the primary controller16. The tank detector circuit output may be time stamped and buffered inthe tank detector circuit 24, the controller 16, or elsewhere in theprimary unit 10, to assist with synchronization of other measurements,where appropriate.

The secondary device 30 is separable from the primary unit 10 and has asecondary coil 32 which couples with the electromagnetic field generatedby the primary unit 10 when the secondary device 30 is in proximity tothe primary unit 10. In this way, power can be transferred inductivelyfrom the primary unit 10 to the secondary device 30 without directelectrical conductive contacts.

FIG. 1 shows one embodiment of a secondary device 30 capable ofreceiving contactless power from the primary unit 10. As it relates tothe reception of contactless power, the secondary device 30 generallyincludes a secondary coil 32, a rectifier 34, a secondary detectioncircuit 36, a controller 38, and a load 40. The secondary coil 32 may bea coil of wire or essentially any other inductor capable of generatingelectrical power in response to the varying electromagnetic fieldgenerated by the primary unit 10. The rectifier 34 converts the AC powerinto DC power. Although not shown, the device 30 may also include aDC-DC converter in those embodiments where conversion is desired. Thecontroller 38 is configured to apply the rectified power to the load 40.In this embodiment, load 40 represents the electronics of the device 30.In some applications, the load 40 may include a battery or other powermanagement circuitry capable of managing the supply of power to theelectronics of the device 30. In alternative embodiments, the controller38 may include power management circuitry. The controller 38 may includecommunication circuitry to enable communication with the primary unit10. The controller 38 may communicate by utilizing the inductivecoupling, for example by using a backscatter modulation scheme, or by anexternal communication path such as an RF transceiver.

The secondary device 30 in the FIG. 1 system includes a secondarydetector circuit 36 connected to the control unit 38. The secondarydetector circuit 36 performs a measurement of a characteristic of theelectrical power delivered to the load 40, in response to a signalprovided by the control unit 38 or an internal clock signal. Thesecondary detector circuit 36 provides an output representative of acharacteristic of the electrical power delivered to the load 40 to thecontrol unit 38. The secondary detector circuit 36 in the currentembodiment includes both an instantaneous current sensor and aninstantaneous voltage sensor. In general, the secondary detectioncircuit 36 may be located anywhere after the AC/DC rectifier 34 in thesecondary device. In other embodiments, the secondary detector circuit26 may include essentially any sensor or sensors capable of measuringone or more characteristics of the power delivered to the load, whichcan be utilized to determine the power delivered to the load. Thesecondary detector 36 communicates its output with the secondarycontroller 38. The secondary detector circuit output may be time stampedand buffered in the secondary detector circuit 36, the controller 38, orelsewhere in the secondary unit 10, to assist with synchronization ofother measurements, where appropriate.

The input detector circuit 24, tank detector circuit 26, and secondarydetector circuit 36 (referred to collectively as detector circuits) mayinclude amplifiers arranged to produce an output signal that is directlyproportional to the current in the input power. The detector circuitsmay also includes band-pass circuitry for removing variation in theoutput signal. The detector circuits may also include an amplifier foramplifying the filtered signal. The detector circuits may also include acomparator for converting the amplifier output to a high or low signal.The various detector circuits, whether they are voltage sensors orcurrent sensors are generally conventional and can be essentially anytype of sensor that obtains the desired measurement. For example, insome embodiments, the current sensors are current sense transformers.Alternatively, they could be shunt resistors, integrated sensors basedon the hall effect, or any other device that transduces current into avoltage measurable by a microcontroller. In one embodiment, thedetectors could be resistor/capacitor voltage dividers.

There are a number of circumstances under which the primary unit mayrestrict or stop the inductive power supply from the primary unit. Someof those conditions include detection of a substantial parasitic load inthe vicinity of the primary unit, no secondary device 30 of the systemis present in the vicinity of the primary unit 10, a secondary device 30is present but does not currently require power. A load does not requirepower, for example, when turned off or when, in the case of arechargeable battery or cell, the battery or cell is fully charged.

In one embodiment, a substantial parasitic load is found where theexpected input current differs from the measured input current by 100mA. The tolerance for parasitic metal can be changed by altering thethreshold difference between expected input current and measured inputcurrent. A higher threshold indicates a higher level of tolerance ofparasitic metal in the field, and a lower threshold indicates a lowerlevel of tolerance of parasitic metal in the field. In the currentembodiment, the 100 mA threshold was selected to ensure that if morethan 1.9 Watts is dissipated into parasitic metal then the systemrestricts or stops inductive power supply.

In other embodiments, the friendly parasitics could be used in thecalculation of expected input current and then would not be a factor indetermining whether parasitic meta 1 is present. In alternativeembodiments that consider friendly parasitics in the expected inputcurrent calculation, the criteria for declaring a substantial parasiticload may be different.

FIG. 2 is a flowchart for use in explaining a method for detecting thepresence of a substantial parasitic load in the vicinity of the primaryunit in accordance with the present invention.

In the current embodiment, from time to time the secondary device sendsa parasitic metal detection packet (PMD packet herein) to the primaryunit so that the primary unit can determine whether there is anyparasitic metal in the field. In the current embodiment, a PMD packet issent to the primary unit every 250 ms. In alternative embodiments, PMDpackets may be sent more or less frequently, or upon request from theprimary unit, or essentially in any other scenario where it isappropriate to determine whether there is parasitic metal in the fieldgenerated by the primary unit.

When it is time to send a PMD Packet 202, the secondary detectioncircuit 36 initiates a voltage measurement and a current measurementafter the AC/DC rectification, alternatively a power measurement couldbe taken using a different sensor system. In alternative embodiments,different measurements at different positions within the secondarydevice may be taken instead of or in addition to these measurements.

Any measurements for the PMD packet are assembled into the payload ofthe packet, along with any other information that may be useful for theparasitic metal detection in the primary unit. For example, the PMDpacket may include time stamp information, indicating the time at whichthe measurements were taken, for synchronization purposes. In addition,the PMD packet may include information about missing data points or ifthere has been any smoothing, averaging, or other weighting function.The PMD packet may also include identifying information about thedevice, for example so that various information could be looked up in alook-up table stored on the primary unit. For example, some primaryunits may include a table of friendly parasitics associated with varioussecondary devices. In some other embodiments, the secondary device maycommunicate its friendly parasitics directly. Although described in thecontext of a PMD packet, it should be understood that the format inwhich the information is sent to the primary unit is unimportant,non-packet communication techniques are also viable. In the currentembodiment, the packet contains a 3 byte payload (5 bytes total)including two 10 bit variables with the most significant bits from bothvariables sharing the last byte. In the current embodiment, the PMDpacket does not include any timestamp information, but instead theprimary unit synchronizes by assuming there was a 9 ms delay from thetime it receives the PMD packet to when the secondary measurements weretaken.

Once the PMD packet is assembled, it is sent to the primary unit 208.Upon receipt of the PMD packet, the primary unit in the currentembodiment obtains the primary coil current measurement that is storedin a buffer that corresponds to the 9 ms before the PMD packet wasreceived. This fixed 9 ms delay accounts for the latency in sending thePMD packet and ensures that the measurements on the primary andsecondary side are synchronized.

The primary unit controller determines the expected input currentutilizing the information received from the secondary device and theappropriate measurements from the primary unit 212. As described above,this determination may be performed based on a pre-defined formula thataccounts for a number of different losses in the system with a best-fitanalysis.

The primary unit also measures the primary input current at thesynchronization time 214. Like the coil current, the primary inputcurrent may be kept in a buffer to assist with synchronization.

The primary measured input current is compared to the primary expectedor calculated input current to determine whether there is any parasiticmetal in the field 216. The difference between the measured inputcurrent and the determined input current represent any unaccounted forlosses in the system. In the current embodiment, the numbers may besimply subtracted and if the difference is greater than a threshold,then the system determines that there is a significant amount ofparasitic metal in the field and the contactless power supply may take anumber of different actions. For example, the system may shut down,lower output power, or turn on warning lights 218. If the difference isless than the threshold, then the system determines that there is not asignificant amount of parasitic metal in the field and the system eitherdoes nothing or indicates that no parasitic metal is present. The systemmay then wait until it is time to send another PMD packet.

In one embodiment, if parasitic metal is detected, the primary unit willremain in the shutdown mode until it is reset in some way. Such a resetcould be manually initiated by a user of the primary unit, oralternatively the control unit 16 could periodically start to supplyinductive power again and repeat the parasitic metal detection todetermine whether to remain in the shutdown mode or not.

The parasitic metal detection techniques described above in connectionwith FIG. 2 may be supplemented by other, different parasitic metaldetection techniques. For example, some parasitic metal detection ismore reliable where the secondary loads can be periodically disconnectedand the system is allowed to be observed in a ring down state. In orderto perform a ringdown parasitic metal detection, all of the secondarydevices in the vicinity of the primary unit are deliberately set to ano-load state. In this no-load state, supply of any of the powerreceived inductively by the secondary device to an actual load thereofis prevented. This allows the system to obtain information about theparasitic metal in the field without having to consider the secondarydevice load. The ability to combine ringdown parasitic metal detectionwith input power parasitic metal detection allows for more accurateparasitic metal detection under some circumstances.

In one embodiment of ringdown, there is an open secondary circuit wherethe coil has its own inductance value and equivalent series resistance.This means it will respond in knowable ways to a variety of stimuli. Ifan impulse is provided, the voltage in the resulting R (ESR) L (Coil) C(tank cap) circuit while decay with a well known time constant. If itdecays quicker, it means that either L went down, or ESR went up, whichsuggests parasitic metal. If we instead turn the coil on at a certainfrequency, again the RLC circuit should have a predictable voltage orcurrent. Again, parasitic metal will shift the response away from theexpected (higher current, lower voltage, etc).

FIG. 3 shows three representative scenarios 300, 302, 304 of therelationship between input power and power consumed in a contactlesspower supply system. In the first scenario 300, the primary unit and thesecondary device are aligned and there is no unfriendly parasitic metalpresent in the field. Put another way, there are no unaccounted forlosses in the first scenario. In this scenario, the combined primarypower losses 306, secondary power losses 308, and power consumed by theload 310 is equal to or substantially the same as the input power 312.In the current embodiment, the primary power losses 306 can includepower loss to primary unit magnetics 314 (such as primary unit shieldinglosses), primary unit electronics 316 (such as rectification, switching,regulation, and filtering losses), and the primary coil 318 (such as I²Rlosses). In the current embodiment, the secondary power losses 308include power losses to friendly parasitics 320 (such as parasitic metalin the secondary device losses), secondary magnetics 322 (such assecondary device shielding kisses), secondary coil 324 (such as I²Rlosses), and secondary electronics 326 (such as rectification,regulation, and filtering losses). When the known power consumption,including power used by the load and power losses in the system, issubstantially the same as the input power, then there are no unaccountedfor losses in the system, such as unknown parasitic metal.

In the second scenario 302, the primary unit and the secondary deviceare misaligned. When the secondary device is misaligned from theinductive power supply, the coupling decreases. This can lead to theprimary delivering more power to the coil so as to deliver the sameamount of power to the secondary. Because more power is delivered, inthe current embodiment the losses increase. For example, there are moreelectronics losses in the primary (Î2*R, rectification if applicable,switching, operating frequency may have shifted), there is more powerlost in the coil due to higher coil current, there is more power lost inthe magnetics due to a larger field being generated by the primary coil,there is more power lost in the parasitics, both friendly and foreigndue to a larger field, and in some circumstances secondary losses mayincrease if the operating frequency shifted, which could cause minorincreases in losses in the secondary rectification and secondary coil.In some embodiments, the changes in these secondary losses can beignored. In general, the power losses in the load and in the secondary(other than rectification and coil) does not change because it is still(trying) to draw the same amount of power. In the illustratedembodiment, additional input power 312 is provided (either by adjustingoperating frequency, resonant frequency, duty cycle, rail voltage, orsome other parameter) in order to deliver he same amount of power to theload 310 as in the first scenario (because the load has not changed).However, because the primary unit and the secondary device aremisaligned, there may be additional primary unit losses 306 andsecondary device losses 308. In the current embodiment, there areincreased losses in primary unit magnetics 315, primary unit electronics317, primary coil 319, secondary device friendly parasitics 321, andsecondary device magnetics 323. In the current embodiment, the powerloss in the secondary coil 324, secondary device electronics 326, andthe load 310 remains constant. If the input power 312 is generally equalto the combined power losses 306, 308 and power consumed by the load310, then there is no unknown power loss in the field such as unknownparasitic metal.

In the third scenario 304, the primary unit and the secondary device arealigned, but a piece of parasitic metal is placed in the field.Additional input power 312 is provided (either by adjusting operatingfrequency, resonant frequency, duty cycle, rail voltage, or some otherparameter) in order to deliver the same amount of power to the load 310as in the first and second scenario (because the load is the same in allthree scenarios). However, because there is a piece of unknown parasiticmetal in the field, there may be additional primary unit losses 306,additional secondary device losses 308, and some unaccounted for powerloss 328. In the current embodiment, there are increased losses inprimary unit magnetics 315, primary unit electronics 317, primary coil319, secondary device friendly parasitics 321, and secondary devicemagnetics 323. In the current embodiment, the power loss in thesecondary coil 324, secondary device electronics 326, and the load 310remains constant. Because there is a significant difference between theinput power 312 and the combined known power losses 306, 308 and powerconsumed by the load 310, the system can assume that there is an unknownpower loss in the field, such as unknown parasitic metal. In response todetecting this unknown power loss the contactless power transfer systemmay restrict or stop power transfer.

The scenarios in FIG. 3 are not drawn to scale, and are merely providedas examples to assist in explanation. Further, the relative amounts ofpower loss and power use by the load shown in these three scenarios 300,302, 304 are merely representative. In some embodiments, there may beadditional or fewer types of power loss. For example, if the secondarydevice does not include any friendly parasitics, then there would not beany associated power loss. The various losses in FIG. 3 are exaggeratedto illustrate how the system can tell the difference betweenmisalignment between a primary unit and secondary device and parasiticmetal being placed in the field. Merely comparing the amount of inputpower to the amount of power delivered to the load does not allow asystem to distinguish between scenario two 302 and scenario three 304.Even if the system accounts for the various losses when the primary unitan secondary unit are aligned, unless the system accounts for changes inpower loss that occur due to misalignment, there is the potential for alarge amount of false positives due to misalignment. If the system canaccount for the changes in power loss during operation, such asmisalignment, the system will incur fewer false positives.

As mentioned above, systems can have trouble distinguishing betweenpower losses due to misalignment vs. power losses due to parasiticmetal, or that do not have sufficient resolution can trigger falsepositives, resulting in restriction or halting of the contactless powersupply to the secondary device. In the current embodiment, the expectedinput current or expected input power match the measured input currentor expected input power because the formula accounts for the losses dueto misalignment during operation.

In addition to having misalignment or parasitic metal added to thefield, it is possible to have both simultaneously. For example, a usermay accidentally throw his keys on to the charging surface, nudging thesecondary device out of place. Under these circumstances, the losses dueto parasitic metal and the losses due to misalignment both increasesimultaneously. Because the system is looking for a relationship betweeninput power and the known losses, the system can still identify thatthere is parasitic metal in the field.

In the current embodiment, the primary coil current, secondary current,and secondary voltage are plugged into a formula that was derived andcoded into the controller at the time of manufacture. The formula isderived by considering known losses based on the fixed impedance andresistance values associated with the primary unit.

An alternative embodiment of a method of controlling inductive powertransfer in an inductive power transfer system, such as the onedescribed above in connection with FIG. 1, is described below. Theinductive power transfer system 100 includes a primary unit 10 and asecondary device 30. The primary unit 10 includes a tank circuit 23 anda switching circuit 14, which together are operable to generate anelectromagnetic field. The system also includes a secondary device 30that is separable from the primary unit and adapted to couple with thefield when the secondary device is in proximity to the primary unit sothat power is received inductively by the secondary device from theprimary unit without direct electrical conductive contacts therebetween.

In one embodiment, the switching circuit operates at an operatingfrequency that varies between a range of different operating frequenciesduring operation. In some embodiments, the operating frequency or someother parameter of the primary unit may be adjusted in response to achange in the load or a request from the load. For example, if thesecondary device demands additional power, the primary unit may adjustthe operating frequency, duty cycle, resonant frequency, or rail voltageto increase its power output. An example of one such primary unit isdescribed in U.S. Pat. No. 7,212,414 to Baarman, filed on Oct. 20, 2003,and is herein incorporated by reference in its entirety. In addition topower loss changing due to misalignment of the primary unit and thesecondary device, power loss may change as a function of the operatingfrequency of the switching circuit or the power demand of the loadassociated with the secondary device. For example a programmableelectronic load may be utilized to test different currents at a constantvoltage or different voltages at a constant current.

The method includes measuring a characteristic of input power in theprimary unit, measuring a characteristic of power in the tank circuit ofthe primary unit, receiving, in the primary unit, information from theat least one secondary device, estimating power consumption in theinductive power transfer system as a function of at least the measuredcharacteristic of power in the tank circuit of the primary unit,comparing the measured characteristic of input power in the primaryunit, the information from the at least one secondary device, and theestimated power consumption to determine there is an unacceptable amountof parasitic metal present in proximity to the primary unit, andrestricting or stopping the inductive power transfer from the primaryunit in response to a determination that the unacceptable amount ofparasitic metal is present in proximity to the primary unit.

Estimating the power consumed can include estimating the power loss inthe inductive power transfer system, estimating power drawn by a load ofthe secondary device, or both. The estimation of power loss in theinductive power transfer system can be a function of the measuredcharacteristic of power in the tank circuit of the primary unit and theinformation from the secondary device. For example, the power lossestimation may include estimating the primary unit magnetic hysteresispower loss, the primary unit magnetic eddy current power loss, theprimary unit voltage power loss, the primary unit resistive power loss,and the secondary device power loss. Information regarding the secondarydevice power loss may be partially or entirely provided by the secondarydevice. For example, the information can be in the form of a secondarydevice ID, a measurement of a characteristic of power in the secondarydevice, an estimation of power loss in the secondary device, one or morepower loss coefficients (including coefficients that characterize themagnetic hysteresis and magnetic eddy losses), or a combination thereof.The secondary device power loss can be described in terms of thesecondary device eddy current power loss, secondary magnetic hysteresisloss, the secondary device voltage power loss, and the secondary deviceresistive power loss.

Additional sensors can be included on the primary unit and secondaryunit to more accurately measure the various power losses in the systemduring operation. However, additional sensors can increase the cost andsize of the primary unit and secondary device. Accordingly, in someembodiments, curve fitting analysis can be utilized to estimate thepower losses based on experimental data. For example, in one embodiment,experimental data can be collected on various combinations of primaryunits, secondary devices, power loads, secondary device positions(including location and orientations), frequency, and friendlyparasitics. The types of data that can be collected experimentally mayinclude essentially any type of measurement. In one embodiment, the datacollected may include input voltage, input current, input powercalculations, primary coil voltage, primary coil current, transmit powercalculations, secondary coil voltage, secondary coil current, receivedpower calculations, output voltage, bridge voltage, output current. Inalternative embodiments, additional, different, or fewer measurements orcalculations may be collected. Any technique for taking the measurementscan be utilized, including but not limited to average values, RMSvalues, power factor, crest factor, peak values, and phase betweenvoltage/current.

In one embodiment, the measured characteristic of input power in theprimary unit, information from the secondary device, and the estimatedpower consumption can be compared to determine there is an unacceptableamount of parasitic metal present in proximity to the primary unit oranother unaccounted for power loss such as a malfunctioning component.This comparison can include a variety of different techniques indifferent embodiments. In one embodiment the comparison entailscalculating the total power consumption based on the characteristic ofinput power in the primary unit, a characteristic of tank circuit power,operating frequency, and the information from the secondary device anddetermining that there is a foreign object present in proximity to theprimary unit by detecting a difference between the calculated totalpower consumption and the estimated power consumption.

In one embodiment, a determination about whether there is anunacceptable amount of parasitic metal present in proximity to theprimary unit can be made by taking the difference between the measuredtotal power consumption and the measured power consumption and comparingthat value to a threshold value. The threshold may be dynamic and basedon the operating point of the system or on a variety of other factors.If the value exceeds the calculated threshold then there is anunacceptable amount of a parasitic metal, if it does not exceed thethreshold then the amount of parasitic metal present is acceptable.

As described above, a best-fit analysis, sometimes referred to as curvefitting, can be performed by sweeping the secondary device through arange of different positions (locations and orientations) with respectto the primary unit. This curve fitting can be distilled intocoefficients for a formula or a set of formulas to determine whetherthere is an unacceptable amount of parasitic metal in the field. Oneexample of a set of formula for this is:

P _(Measured) =C ₀+(C ₁ +C ₂)i _(tx) _(—) _(coil) ^(f)+(C ₃ +C ₄)·(i_(tx) _(—) _(coil) ^(f))² +C ₅ i _(tx) _(—) _(input) +C ₆ i _(tx) _(—)_(coil) ²+C₇ i _(rx) _(—) _(rectified) +C ₈ i _(rx) _(—) _(rectified)²+C10

P _(Calc)=Pin_(ts) −C _(p) ·P _(rx) _(—) _(rectified)

P _(Foriegn) =P _(Calc) −P _(Measured)

Each of the coefficients can be determined experimentally. Thecoefficients can be determined individually based on the individual typeof power loss through physical observation of components in the system,or a curve fit may be performed to obtain all of the coefficientssimultaneously. In one embodiment, a brute force curve fit using knownmultivariate polynomial regression techniques or other methods offitting observed data to a given formula may be utilized to determinethe coefficients. These techniques are generally referred to as curvefitting. In another embodiment, the coefficients are determinedexperimentally by collecting data as a check for bench-measuredparameters, such as equivalent series resistance and voltage drop. Thedata can be collected for a variety of different combination of primaryunits, secondary devices, loads, positions, friendly parasitics. Txrefers to the primary unit or transmitter, and Rx refers to thesecondary device or receiver. The i_(tx) _(—) _(coil) refers to theprimary unit coil current, i_(tx) _(—) _(input) refers to the primaryunit input power, and i_(rx) _(—) _(rectified) refers to the currentafter rectification in the secondary device. In the current embodiment,the coefficients are the same for all loads.

Although the current embodiment includes 10 coefficients, in alternativeembodiments additional, different, or fewer coefficients may beutilized. For example, the C7 and C8 coefficients may be eliminatedwithout significantly affecting the parasitic metal detection successrate. Further, although a set of formulas is provided above to helpunderstand the approach, a single formula can be generated bysubstitution. The formula compares the input power, the output power,and the various losses in the system in order to determine if there isany parasitic metal in proximity of the primary unit. Tx refers to theprimary unit or transmitter, and Rx refers to the secondary device orreceiver. The i_(tx) _(—) _(coil) refers to the primary unit coilcurrent, i_(tx) _(—) _(input) refers to the primary unit input power,and i_(rx) _(—) _(rectified) refers to the current after the rectifierin the secondary device, for example as shown in the current sensor ofFIG. 1.

In one embodiment, the power loss estimation can be made more accurateby accounting for frequency variation. For example, some inductive powersupplies vary the operating frequency of the switching circuit of theprimary unit during operation. This change in operating frequency canhave an effect on how much power loss there is in the system. In orderto more accurately estimate the power loss in the system, for eachoperating frequency, an equivalent series resistance value of theprimary coil can be determined. The data points of equivalent seriesresistances can be curve fit in order to determine an eddy current powerloss coefficient for estimating primary unit eddy current power loss.That is, it can be difficult to calculate the eddy current power lossesthat occur for any given primary unit and secondary device at any givenfrequency. By determining experimentally what the eddy current powerloss is for a combination of primary units and secondary devices at aplurality of frequencies, a generalized function of what losses toexpect can be developed as a function of frequency. This same techniquecan be utilized for any power loss in the system that is dependent onfrequency. That is, by collecting data on the different types of powerlosses that occur in various combinations of primary units and secondarydevices at the various frequencies, if the power losses vary based onfrequency, then curve fitting can help to develop a set of coefficientsfor a formula that can be used during operation in order to betterestimate the power losses. All ten coefficients can be determinedexperimentally so that they can be stored in the primary unit orsecondary device during manufacture. In general, primary parameters arehardcoded into the primary unit, while secondary device parameters arecommunicated during operation or during an initialization calibrationroutine. To be clear, this calibration routine can be a separate processfrom the calibration process described herein for determining thecoefficients. Instead, this calibration routine can be utilized toensure that the processing unit that ultimately determines the whetherthere is parasitic metal in the field has access to all of theappropriate coefficients.

The function can be based on classically known loss models for certainparts of the system. For example, magnetic materials have losses relatedto i*f that are due to hysteresis (C1 for primary material, C2 forsecondary material), magnetic materials have losses related to (i*f)̂2that represent eddy current losses in those materials (C3 for pri, C4for sec), capacitors, coils, and FETs have “resistive losses” thatrelate to î2 (C6, C8), since the total power loss on the secondary canbe approximated as a linear function of the received power, we can use ascalar function of received power (C9, C10) to approximate additionallosses. Since we know that these are the physical loss models of all ofthese major circuit components, we can do a multivariate polynomialregression on all of our data collected to find what each of thesecoefficients might be. An alternative way to find the coefficients is toincrementally measure them in system and determine their values. Thiscan then be verified against the observed (collected) data.

Referring to the formula above, C0 represents the primary unit offset,it can be utilized during the curve fitting process to represent alllosses not dependent on current. As mentioned above, all coefficients,including C0, could be best-fit simultaneously (multivariate polynomialregression). Or, if determined experimentally by measuring values ofESR, voltage drop, etc, and C0 would be whatever is left over. Thisrepresents a base level of power that (what the micro uses, etc) thatdoes not vary with load.

C1 and C2 are the coefficients that represent the primary unit andsecondary device magnetic hysteresis losses in the system respectively.In some embodiments the coefficients for the magnetic hysteresis lossesmay be assumed to be zero or near zero and therefore be eliminated fromthe calculation. The primary unit and secondary device can be designedto minimize the hysteresis losses and thereby simply the calculation. IfC3 and C4, as described below, are not constant versus frequency, thenC1 an C2 may be considered nonzero. When ESR is measured, an assumptionis made of the Î2*R loss model. If it fits the function (I*f)̂2*R, thatmeans that when we measure ESR over many frequencies and divide by f̂2,the resultants should be equal to each other (or very close) for allmeasurements of ESR. If they are not, then a different loss model isconsidered and a polynomial regression on (i*f) may be performed.

C3 and C4 represent the primary unit magnetics eddy current loss and thesecondary unit magnetics eddy current loss respectively. In order todetermine C3 and C4, the equivalent series resistance (“ESR”) of theprimary and secondary coils is measured, independent of any magneticmaterial. This is referred to as the bare coil ESR. The bare coil ESRcan be measured over a range of frequencies, for example in the currentembodiment the bare coil ESR is measured over a frequency range of 110kHz to 205 kHz.

In order to determine C3, the equivalent series resistances (“ESR”) ofthe primary coil, primary unit shield (if any), and primary unit magnet(if any) is measured. This is referred to as the primary coil assemblyESR. The bare coil ESR is subtracted from the primary coil assembly ESRin order to provide the ESR of just the primary unit shield and primaryunit magnet, sometimes referred to as the primary magnetics ESR. Ofcourse, these ESR values may be obtained in different ways other thanthe method of the current embodiment. In one embodiment, C3 isdetermined by dividing the primary magnetics ESR by the square of thefrequency at which the measurement is taken. In an alternativeembodiment, in order to increase the accuracy of C3, multiplemeasurements at different frequencies over the expected operating rangecan be taken. In that case, the primary magnetics ESR at each frequencyis divided by the frequency squared and the average of all of thosevalues is taken to account for experimental errors and is deemed to beC3.

In order to determine C4, the ESR of the system is measured. In thecurrent embodiment, the system ESR is measured while the primary unitand the secondary device are aligned and when the system includes theprimary coil, the primary unit shield, the primary unit magnet, the gapbetween the primary unit and the secondary device, the secondary devicecoil, the secondary device shield, the secondary device magnet, and anyfriendly parasitic metal. In the current embodiment, when the system ESRis measured, it is observed from the perspective of the primary coil.The primary unit assembly ESR (primary coil with magnet and primaryshield present) is subtracted from the system ESR to obtain thesecondary device assembly ESR. In one embodiment, C4 is determined bydividing the secondary device assembly ESR by the square of thefrequency at which the measurement is taken. In an alternativeembodiment, in order to increase the accuracy of C4, multiplemeasurements at different frequencies over the expected operating rangecan be taken. In that case, the secondary device assembly ESR at eachfrequency is divided by the frequency squared and the average of all ofthose values is taken to account for experimental errors and is deemedto be C4.

In one embodiment, C4 can be determined more accurately by alsoobtaining the secondary device assembly ESR at a variety of differentalignments. That is, the secondary device may be moved aroundautomatically in space, for example utilizing an X, Y, Z table or XYZtable with rotation to obtain ESR measurements at precise locations andorientations. In the current embodiment, all of the ESR/f̂2 values foreach combination of operating frequency and position can be averaged toobtain a C4 value for a particular secondary device, for example amobile phone with a secondary coil for receiving an electromagneticfield, a magnetic shield, and friendly parasitics such as the housing,electronics, and battery.

One embodiment of a geometric positioning system is shown in FIG. 4, anddesignated 400. The geometric positioning system 400 can be used to movethe relative position of the primary and secondary coil. In the currentembodiment, the geometric positioning system 400 includes a base 402 anda positioner 404. The base 402 can be integrated with a primary unit orportion of a primary unit such as a primary coil 406, or in someembodiments a primary coil 406 or primary unit may be removablyattachable to the base 402. A variable gap 408 may be provided bystacking various different size spacers that does not effect theinductive power transfer. Alternatively, the positioner 404 may provideprecise x,y, and z positioning in space to provide the gap 408 insteadof using a spacer. Foreign objects 410, secondary coils 412, friendlyparasitics 414, and other items may be removably attached to thepositioner 404. Because the current embodiment utilizes circular coils,the positioner 404 only provides movement in a single direction, byincreasing the size of the spacer, the vertical position can be varied.In an alternative embodiment, a multi-axis table can be utilized to movethe coils. Even with a single direction positioner, location of movementcan be provided in the form of a triplet, the X and Y values are alwaysthe same, because movement is diagonal on the XY axis with respect tothe primary coil. The Z component of the triplet is provided by thethickness of the gap 408.

Although one method of determining C3 and C4 is described above, othermethods may also be employed. Any known technique can be utilized tosolve or estimate C3 and C4 in the formula:

I _(tx) ₂ *ESR _(tx-bare) +I _(rx) ₂ *ESR _(rx-bare)+(C ₃ +C ₄)*(i _(tx)*f)²=Input_(power)−(TX _(power) +RX _(power)+Foreign Object_(power))

This formula should hold true for all loads in all positions.

Referring back to the generalized equation for determining whether thereis an unacceptable amount of parasitic metal in the field, C5 and C6represent the electronics losses on the primary unit. The primary unitlosses can be calculated generally determined by subtracting the amountof power in the primary unit coil from the amount of input power.However, during operation, it can be expensive to include the hardwareto take these power measurements. Further, in some embodiments, theprimary unit electronics power losses can be accurately estimatedutilizing some curve fit coefficients, primary unit input current, andprimary unit coil current. In alternative embodiments, differentmeasurements may be utilized to estimate the primary unit electronicslosses. For example, in some embodiments, the losses can be fit to asecond order equation based solely on the primary unit coil current. Inthat embodiment, C5 is a first order term and C6 is a second order termplus the primary unit bare coil ESR.

C7 and C8 represent losses in the secondary device rectifier. Theselosses can be curve fit to a second order polynomial equation ofP_(recieved)−P_(rectified)=C₇*i_(rx-rect)+C₈*i_(rx-rect) ₂ .

In the current embodiment, for simplicity, the losses in the secondarydevice coil can be included in C8 by adding the secondary device barecoil ESR to C8. In alternative embodiments, the losses in the secondarydevice coil can be a separate term. It is worth noting that if aslope-intercept resistor network is utilized on the secondary device, C7and C8 can be assumed to be zero and coefficients C9 and C10 would beused instead. In order to utilize the C7 and C8 terms in the currentembodiment, a measurement during operation of the secondary coil currentis necessary. By removing those terms, the calculation may still besufficiently accurate, but the additional complexity in hardware designcan be eliminated. Instead of C7 and C8, a C9 coefficient may beutilized to estimate the amount of secondary device power loss.

C9 and C10 can be calculated to together. In one embodiment, plottingPreceived−Pdelivered against load, allows a straight line through thecurve to approximate loss to be drawn. The intercept is C10 andrepresents the secondary device power loss offset. C9 is 1+the slope,since the C9 term also accounts for the power delivered to the load.

One embodiment of a method for calibrating an inductive power transfersystem utilizing a calibration system is described below.

The calibration system may include test equipment for performing powertests and thermal tests. The power test equipment may include ageometric positioning system 400, 2 AC current probes, 2 DC currentprobes, and analog to digital converters having 10 Channels of 10 MHZ+12bit+sampling. The analog to digital converters can be located at variouslocations in the circuit for sampling fast enough and with enoughresolution to determine desired information. The test equipment mayinclude software that sweeps through a number of positions on thegeometric positioning system and samples all of the data points. Thedata points can be sampled at a rate of 1 second, or some other rate.The raw data can be saved or in the alternative the raw data may bediscarded once certain calculated values are obtained. The variousvalues that can be recorded during the calibration were discussed above,suffice it to say the values can include various voltage, current,frequency, and phase measurements throughout the system. The thermaltest equipment may be utilized to determine the threshold for the amountof acceptable parasitic metal.

In one embodiment of the method of calibration, different combinationsof receiver coils, loads, friendly parasitics, locations, andtransmitters can be tested. For example, a variety of different shapesand sizes of secondary coils that are designed for a particular amountof power may be calibrated. A variety of different loads, for examplestatic loads of 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4, 5W at each location canbe tested. Alternatively or additionally, actual operational devicessuch as specific models of mobile phones or other secondary devices maybe tested. Additionally, unloaded configurations can be tested. Avariety of different friendly parasitics can be tested. For example,materials such as an aluminum plate, copper plate, stainless steelsheet, blue steel sheet, or the specific body of a mobile phone. In thecurrent embodiment, each of the friendly parasitics are 25% bigger thanthe secondary coil, meaning that it extends past the edges of thesecondary coil by 25% in each direction. A variety of differentpositions in space, or in separation and misalignment can be measured.In the current embodiment, the orientation is kept constant, but 5different locations are measured. A variety of different primary unitsor primary coils within primary units can be tested.

The method of calibrating includes placing the secondary device in aplurality of different positions with respect to the primary unit,operating the primary unit at a plurality of different loads, for eachposition and load combination, determining an equivalent seriesresistance value of the secondary device, for each load operatingfrequency and load combination, determining an equivalent seriesresistance value of the primary unit, curve fitting the determinedequivalent series resistance values of the secondary device to determinean secondary device eddy current power loss coefficient for estimatingsecondary device eddy current power loss, and curve fitting thedetermined equivalent series resistance values of the primary unit todetermine a primary unit eddy current power loss coefficient forestimating primary unit eddy current power loss. It is worth noting thatbecause the primary unit in the current embodiment adjusts operatingfrequency in dependence on the load present in the system, by changingthe load or the position of the load, the primary unit changesfrequency. These changes in frequency can affect the power losses in thesystem.

Although the above embodiment are described in connection with a singlesecondary device, during operation or calibration multiple secondarydevices may be accounted for. That is, the formula and coefficients canbe expanded to account for all known secondaries in the system and thelosses associated with those secondaries. For each secondary device,each of the secondary device power losses is duplicated.

The above description is that of current embodiments of the invention.Various alterations and changes can be made without departing from thespirit and broader aspects of the invention as defined in the appendedclaims, which are to be interpreted in accordance with the principles ofpatent law including the doctrine of equivalents. Any reference to claimelements in the singular, for example, using the articles “a,” “an,”“the” or “said,” is not to be construed as limiting the element to thesingular. It is to be understood that the invention disclosed anddefined herein extends to all alternative combinations of two or more ofthe individual features mentioned or evident from the text and/ordrawings. All of these different combinations constitute variousalternative aspects of the present invention.

1. A method of controlling inductive power transfer in an inductivepower transfer system, the system including a primary unit, having atank circuit and switching circuit, operable to generate anelectromagnetic field and at least one secondary device, separate fromthe primary unit, and adapted to couple with the field when thesecondary device is in proximity to the primary unit so that power isreceived inductively by the secondary device from the primary unitwithout direct electrical conductive contacts therebetween, the methodcomprising: measuring a characteristic of input power in the primaryunit; measuring a characteristic of power in the tank circuit of theprimary unit; receiving, in the primary unit, information from the atleast one secondary device; estimating power consumption in theinductive power transfer system as a function of at least the measuredcharacteristic of power in the tank circuit of the primary unit;comparing the measured characteristic of input power in the primaryunit, the information from the at least one secondary device, and theestimated power consumption to determine there is an unacceptable amountof parasitic metal present in proximity to the primary unit; andrestricting or stopping the inductive power transfer from the primaryunit in response to a determination that the unacceptable amount ofparasitic metal is present in proximity to the primary unit.
 2. Themethod as claimed in claim 1 wherein the estimating power consumptionincludes at least one of estimating power loss in the inductive powertransfer system, estimating power used by a load of the secondarydevice, or a combination thereof.
 3. The method as claimed in claim 1wherein the estimating power consumption includes estimating power lossin the inductive power transfer system as a function of the measuredcharacteristic of power in the tank circuit of the primary unit and theinformation from the secondary device.
 4. The method as claimed in claim1 wherein the switching circuit operates at an operating frequency thatvaries between a range of different operating frequencies duringoperation and estimating power consumption includes estimating powerloss in the inductive power transfer system as a function of themeasured characteristic of power in the tank circuit of the primaryunit, the information from the secondary device, and the operatingfrequency of the switching circuit.
 5. The method as claimed in claim 1wherein estimating power consumption includes estimating primary unithysteresis power loss; estimating primary unit eddy current power loss;estimating primary unit voltage power loss; estimating primary unitresistive power loss; and estimating secondary device power consumption.6. The method as claimed in claim 5 wherein estimating secondary devicepower consumption includes: estimating secondary device eddy currentpower loss; estimating secondary device voltage power loss; andestimating secondary device resistive power loss.
 7. The method asclaimed in claim 5 wherein estimating secondary device power lossincludes estimating secondary device eddy current power loss andestimating secondary device hysteresis as a function of the measuredcharacteristic of power in the tank circuit of the primary unit.
 8. Themethod as claimed in claim 1 wherein the information from the at leastone secondary device includes a secondary device ID, a measurement of acharacteristic of power in the secondary device, an estimation of powerloss in the secondary device, one or more power loss coefficients, or acombination thereof.
 9. The method as claimed in claim 1 wherein theinformation from the at least one secondary device includessynchronization information for synchronizing the comparing the measuredcharacteristic of input power in the primary unit, the information fromthe at least one secondary device, and the estimated power consumption.10. The method as claimed in claim 1 wherein the characteristic of inputpower includes a current or voltage in the primary unit before theswitching circuit and the tank circuit.
 11. The method as claimed inclaim 1 wherein the characteristic of power in the tank circuit includesa current or voltage in the tank circuit.
 12. The method as claimed inclaim 1 wherein comparing the measured characteristic of input power inthe primary unit, information from the at least one secondary device,and the estimated power consumption to determine there is anunacceptable amount of parasitic metal present in proximity to theprimary unit includes: calculating the total power consumption based onthe characteristic of input power in the primary unit and theinformation from the at least one secondary device; and determining thatthere is a foreign object present in proximity to the primary unit bydetecting a difference between the calculated total power consumptionand the estimated power consumption.
 13. The method as claimed in claim1 including determining that the unacceptable amount of parasitic metalis present in proximity to the primary unit when the difference betweenthe calculated total power consumption and the estimated powerconsumption exceeds a threshold value.
 14. The method as claimed inclaim 1 including: placing the secondary device in a plurality ofdifferent positions with respect to the primary unit; for each position,determining an equivalent series resistance value of the secondarydevice; determining an eddy current power loss coefficient forestimating secondary device eddy current power loss based on theequivalent series resistance values of the secondary device.
 15. Themethod as claimed in claim 1 including: placing the secondary device ina plurality of different positions with respect to the primary unit andoperating the primary unit at a plurality of different operatingfrequencies; for each position and operating frequency combination,determining an equivalent series resistance value of the secondarydevice; determining an eddy current power loss coefficient forestimating secondary device eddy current power loss based on theequivalent series resistance values of the secondary device.
 16. Themethod as claimed in claim 15 wherein determining an equivalent seriesresistance value of the secondary device includes measuring anequivalent series resistance of the primary unit alone, measuring anequivalent series resistance of the inductive power transfer system, andsubtracting the equivalent series resistance of the primary unit alonefrom the equivalent series resistance of the inductive power transfersystem to determine the equivalent series resistance value of thesecondary device.
 17. The method as claimed in claim 15 wherein theprimary unit includes a primary unit shield, a primary unit magnet, andthe primary unit tank circuit includes a primary unit coil, thesecondary device includes a secondary coil, a secondary shield, andsecondary friendly parasitic metal.
 18. The method as claimed in claim 1including: placing the secondary device in a plurality of differentpositions with respect to the primary unit; operating the primary unitat a plurality of different operating frequencies; connecting aplurality of different loads to the secondary device; for each position,operating frequency, and load combination, determining an equivalentseries resistance value of the secondary device; determining an eddycurrent power loss coefficient for estimating secondary device eddycurrent power loss based on the equivalent series resistance values ofthe secondary device.
 19. A method for designing an inductive powertransfer system with power accounting comprising: providing a primaryside with a tank circuit for transferring contactless energy; providinga secondary side including a secondary coil for receiving thecontactless energy and a load in electrical communication with a load;changing the distance between the primary side and the secondary side;changing the load of the secondary side; for a plurality of distancesbetween the primary side and the secondary side and for a plurality ofloads, measuring at least one circuit parameter on the primary side inthe tank circuit during the transfer of contactless energy; for aplurality of distances between the primary unit and the secondary deviceand for a plurality of loads of the secondary device, measuring at leastone circuit parameter on the secondary side during the transfer ofcontactless energy; selecting a formula to describe power consumption inthe system during the transfer of contactless energy based on aplurality of coefficients, the at least one circuit parameter on theprimary side in the tank circuit, and the at least one circuit parameteron the secondary side; and determining the coefficients using themeasured circuit parameters on the secondary side and the measuredcircuit parameters on the primary side.
 20. The method of claim 19wherein determining the coefficients of the formula includes determiningthe coefficients based on the type of power loss through physicalobservation of components in the system.